linux-kernel - Re: Scheduler benchmarks

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Message-ID: <20070918084825.GA25803@elte.hu>
Date:	Tue, 18 Sep 2007 10:48:25 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Rob Hussey <robjhussey@...il.com>
Cc:	linux-kernel@...r.kernel.org, ck@....kolivas.org
Subject: Re: Scheduler benchmarks - a follow-up

* Rob Hussey <robjhussey@...il.com> wrote:

> The obligatory graphs:
> http://www.healthcarelinen.com/misc/benchmarks/BOUND_NOPREEMPT_lat_ctx_benchmark.png
> http://www.healthcarelinen.com/misc/benchmarks/BOUND_NOPREEMPT_hackbench_benchmark.png
> http://www.healthcarelinen.com/misc/benchmarks/BOUND_NOPREEMPT_pipe-test_benchmark.png

btw., it's likely that if you turn off CONFIG_PREEMPT for .21 and for 
.22-ck1 they'll improve a bit too - so it's not fair to put the .23 
!PREEMPT numbers on the graph as the PREEMPT numbers of the other 
kernels. (it shows the .23 scheduler being faster than it really is)

> A cursory glance suggests that performance wrt lat_ctx and hackbench 
> has increased (lower numbers), but degraded quite a lot for pipe-test. 
> The numbers for pipe-test are extremely stable though, while the 
> numbers for hackbench are more erratic (which isn't saying much since 
> the original numbers gave nearly a straight line). I'm still willing 
> to try out any more ideas.

the pipe-test behavior looks like an outlier. !PREEMPT only removes code 
(which makes the code faster), so this could be a cache layout artifact. 
(or perhaps we preempt at a different point which is disadvantageous to 
caching?) Pipe-test is equivalent to "lat_ctx -s 0 2" so if there was a 
genuine slowdown it would show up in the lat_ctx graph - but the graph 
shows a speedup.

	Ingo
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